Encrypts text using the rot13 cipher.

rot13.c

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>

char *convert_to_rot13(char *str) {
    size_t i;
    for (i = 0; i < strlen(str); i++) {
        if (str[i] > 64 && str[i] < 91) {
            str[i] += str[i] < 78 ? 13 : -13;
        } else if (str[i] > 96 && str[i] < 123) {
            str[i] += str[i] < 110 ? 13 : -13;
        }
    }
    return str;
}

void die(char *error)
{
    if (errno) {
        printf("%d\n", errno);
    } else {
        printf("%s\n", error);
    }
    exit(-1);
}

int main(int argc, char **argv)
{
    FILE *ifp, *ofp;
    char word [100];

    if (argc != 3) {
        die("This script requires only two arguments.");
    } else {
        ifp = fopen(argv[1], "r");
        ofp = fopen(argv[2], "w");

        if (ifp == NULL) {
            die("Error opening input file!");
        } else if (ofp == NULL) {
            die("Error opening output file!");
        }

        while (fgets(word, 100, ifp) != NULL) {
            fprintf(ofp, "%s", convert_to_rot13(word));
        }
    }
}